On Monday, Mr. Volcker, a former Federal Reserve chairman who almost single-handedly rescued the United States from the stagflation crisis of the late 1970s, wrote a 2,000-word rebuke to critics of the Volcker Rule. The rule, which is part of the Dodd-Frank Act, was Mr. Volcker’s brainchild. It seeks to prevent banks from making speculative bets with shareholder money that could put themselves — and taxpayers — at risk.

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The Volcker Rule is a noble and thoughtful effort to make the banking system safer in the long-term postfinancial crisis. Critics in the banking industry, however, say the new regulation comes with many embedded costs for the national and global economy.

Here’s where Mr. Volcker and I differ. He says: “Not so.” I say: “C’mon. It’ll cost the economy, at least in the short term.”

The Volcker Rule may make sense for the long-term stability of the banking system, and this column has been supportive of the rule and its rationale. But to pretend that it is without significant costs, at least here and now, is disingenuous. The rule may be the right thing to do, but let’s admit the truth: it won’t be cheap.

If the costs of the Volcker Rule were so de minimis, as Mr. Volcker has argued, you wouldn’t see regulators, banks and even individuals submitting letters to the Federal Reserve up until the deadline for comments on Monday expressing serious concerns about the effect of the rule.

Jamie Dimon, the chairman and chief executive of JPMorgan Chase, went so far as to belittle Mr. Volcker on Monday. “Paul Volcker by his own admission has said he doesn’t understand capital markets. He has proven that to me,” Mr. Dimon told Fox Business. (Ouch, that was harsh.)

As this column recently noted, a number of foreign regulators have also expressed particular concern about a provision of the Volcker Rule that could keep banks from helping clients buy and sell foreign sovereign bonds — which could, in turn, make borrowing costs for foreign countries higher.

Some industry critics of the Volcker Rule say that removing big commercial banks from making their own bets will remove liquidity from the system, thereby driving up costs of everything from equities to corporate bonds. But the real debate has moved to the rules for banks involving “market making” — finding and matching buyers and sellers. The question is whether it will become so complex under the new legislation, that it will further push up trading costs for customers.

An industry study estimated that the Volcker Rule could cost companies and investors more than $350 billion. That projection, which has been criticized by some economists, could be wildly exaggerated. But even half that number has to be considered substantial.

Mr. Volcker, in his letter, doesn’t buy the arguments. “The restrictions on proprietary trading by commercial banks legislated by the Dodd-Frank Act are not at all likely to have an effect on liquidity inconsistent with the public interest,” he wrote. He also said that there should not be “a presumption that ever more market liquidity brings a public benefit.”

Yet Mr. Volcker doesn’t offer any explanation for why it won’t, except to argue that less liquidity might tamp down speculative trading.

“We are a store,” Mr. Dimon said as a way of rebuttal on Monday on Fox. “Remember when the client calls JPMorgan, if we don’t give them the best price, we don’t get the business. But the best price is a huge benefit to them.”

At issue is this: Can a bank buy a series of bonds, for example, and keep some of them on their balance sheet — or to use Mr. Dimon’s analogy, on the bank’s store shelves — until they can find a customer?

The way the Volcker Rule is currently construed, a bank can’t keep any merchandise on its shelves. It needs to match a buyer and seller; the bank is simply the middleman. The problem with this becomes clear when a banking client asks for an odd lot of a certain financial product. Historically, the bank could buy, say, 1,000 bonds and hand over the 889 that its client had requested. The 111 other bonds would sit on the firm’s balance sheet until it could parcel them out to other clients who wanted to buy them.

Now, such trades may become impossible — or at least, impossibly expensive. Regulators will be watching to make sure that banks aren’t trying to mask proprietary trades as market-making. In other words, regulators are nervous that clever traders might try to buy certain financial products hoping that they increase in value and then selling them — but claiming that the products had really been purchased to help a client. Understanding the intent of each trade is going to become part of the regulatory process.

“We are going to have to have a lawyer, compliance officer, doctor to see what their testosterone levels are, and a shrink, what is your intent?” Mr. Dimon said.

Mr. Volcker is particularly concerned about how banks may try to game the system. “Various arbitrage strategies, esoteric derivatives and structured products will need particular attention, and to the extent that firms continue to engage in complex activities at the demand of customers, regulators may need complex tools to monitor them,” he wrote. Clearly, such trading is going to end up costing banks more, which in turn, will most likely pass those costs onto clients. (The rule is also going to cost Wall Street jobs, which some people, perhaps unfairly, don’t seem to think is a bad thing.)

Finally, he also rebutted critics who say that the Volcker Rule is going to make the United States banking business less competitive with foreign rivals who don’t have such restrictions. Mr. Volcker calls such criticism “superficial,” saying: “Competition in banking, here as elsewhere, is desirable for the benefits it brings in institutional efficiency and better, more economical service to customers. Any contribution of proprietary trading to customer service and competition is not at all obvious. In fact, because of the risks, the conflicts of interest and the adverse cultural influence it may well impede effective competition.”

On the surface, Mr. Volcker would seem right. But if you dig deeper into the complex needs of global corporations that are the clients of big banks these days, they sometimes seek banks to make proprietary bets to help them. In such cases, companies may now look to overseas firms.

Having said all of that, even after adding up all of the costs, the Volcker Rule still makes a whole lot of sense. In the long term, it will prevent banks from taking outsize bets and putting taxpayers at risk. But just remember, this change doesn’t come without a price.